∵ quadrilateral ABCD is a rectangle.

∴∠ABC=90

Similarly, the diagonal of the rectangular ABCD is the diameter of circle 0.

∴AC=50cm

∴△ABC is Rt△

Derived from Pythagorean theorem

AB？ +BC？ =AC？

∴BC？ =AC？ -AB？

That's BC? =2500-x？

∴ y = 2500-x under the root sign?